1. Field of the Invention
The present invention generally relates to temperature sensing circuits and, in particular, relates to such circuits as used with semiconductor junction temperature probes.
2. Statement of the Prior Art
Electrical temperature probes and the electrical sensing circuitry for the operation thereof have been well known and widely used. One of the applications of these probes is microwave ovens. In microwave ovens, the cooking is done very quickly requiring careful control of the cooking cycle. Achievement of this control is attempted by inserting a temperature probe into the food or item being heated and shutting off the oven when the desired temperature is reached. This operation requires a temperature probe having a quick temperature response and a high degree of accuracy over a wide range of normal cooking temperatures.
The most common form of electrical temperature probe or element used is the thermistor, the resistance of which is dependent upon its temperature. Unfortunately, thermistors suffer from the limitation that the resistance response thereof is exponential with respect to the probe temperature. This presents certain sensing circuitry requirements and/or limits to the usefulness of the thermistor in sensing and measuring temperatures beyond limited temperature ranges. Various circuitry which has been developed for the purpose of expanding the usefulness of thermistor temperature sensors are shown and described in U.S. Pat. Nos. 3,882,725; 4,060,715; and 4,106,341. The circuitry described in these patents is capable of linearizing the response of the thermistor temperature sensor, but only at the cost of additional circuitry complexity.
Another form of temperature probe or element, which was developed more recently than the thermistor, but which is also widely used, is the semiconductor junction temperature sensor. This sensor comprises a simple bipolar semiconductor junction, the electrical characteristics of which vary according to the temperature thereof. Two examples of the use of such sensors are U.S. Pat. Nos. 3,808,469 and 3,604,957. The U.S. Pat. No. 3,808,469 patent discloses means for changing the temperature coefficient of the sensor from a negative coefficient to a positive coefficient by the inclusion of a number of positive temperature coefficient elements in the circuit. The U.S. Pat. No. 3,604,957 patent discloses a voltage balancing circuit which compares the temperature of the sensing probe with the temperature of a reference probe. Unfortunately, this circuit is not adapted for allowing variation of the reference temperature.